Overhaul commands handling and make settings handling consistent

[natevw]

The fix for #15 turned out to be fairly involved. I think the is_outdated pull request that's been incorporated into 0.4.1 must have been quite stale, since it not only used different BROWSERIFY settings but also relied on SubProcessCompiler.execute_command behavior that got changed in django-pipeline over a year ago now: https://github.com/jazzband/django-pipeline/commit/1f6b48ae74026a12f955f2f15f9f08823d744515

I ended up basically overhauling this library, sorry 😬

• added a new simple_execute_command helper to replace what django-pipeline lost, but in a much simpler/safer way
• unified handling of settings between the compile_file and is_outdated methods
• deprecated the old string-based BROWSERIFY_ARGUMENTS and BROWSERIFY_VARS — they still work but will log if warnings are enabled
• documented the new more explicit/safer BROWSERIFY_ARGS and BROWSERIFY_ENV settings
• also cleaned up some logging that was in there so that it only happens if self.verbose is set

Apologies that this ended up being such a major and invasive change, but hope it's a valuable contribution nonetheless!

[j0hnsmith]

@natevw I haven't used browserify for years and don't even have a dev env for this repo, can I add you as a collaborator to this repo?

[natevw]

@j0hnsmith Sure, I don't mind. What's the process for releasing on PyPi?

[j0hnsmith]

What's the process for releasing on PyPi?

Just bump the version and run a couple of setuptools (or something?) commands, I'll figure it out at the weekend (or before if I get a chance).

[natevw]

Friendly reminder that it'd be great to have these recent changes released at least in beta form. They've been Working on My Machine™ this week at least.

[j0hnsmith]

done

[natevw]

@j0hnsmith I see this and #19 were published as django-pipeline-browserify==0.6.0, thanks!